Scheme to achieve robustness to electromagnetic interference in inertial sensors

ABSTRACT

A capacitive sensor system and method resistant to electromagnetic interference is disclosed. The system includes a capacitive core, differential amplifier with inverting and non-inverting inputs, capacitive paths, and chopping system. Core can include inputs and outputs coupled to variable capacitors, and common nodes coupling variable capacitors. Capacitive paths couple core outputs to amplifier inputs. When chopping system is high, one polarity voltage is applied to core inputs, a first core output is coupled to the inverting input and a second core output is coupled to the non-inverting input. When the chopping system is low, opposite polarity voltage is applied to core inputs, and core output to amplifier input couplings are flipped. Capacitive paths can include bond wires. Chopping system can be varied between high and low at frequencies that smear noise away from a frequency band of interest, or that smear noise substantially evenly across a wide frequency range.

BACKGROUND OF THE INVENTION

This patent relates to capacitive transducers, and more particularly totechniques for overcoming electromagnetic interference in capacitivesensors.

In inertial sensors, electromagnetic disturbance or interference (EMI)occurs primarily due to capacitive coupling between bond wires andnearby cables, plates, circuitry, etc. FIG. 1 illustrates an exemplaryscenario of EMI. In FIG. 1, a microelectromechanical structure (MEMS)device 102 is coupled to an application specific integrated circuit(ASIC) 104 by a plurality of bond wires 106. A source of EMI 110 that isnear the bond wires 106 creates capacitive coupling 110 between the EMIsource 110 and the bond wires 106. Capacitor symbols are shown in FIG. 1to illustrate the capacitive coupling 110, but this simply illustratesthe parasitic capacitance between the electromagnetic disturbance source110 and the bond wires 106, no actual electrical component is present.The bond wires coupling capacitive nodes are the most sensitive to EMI,as opposed to nodes driven by a voltage source or amplifier.

In environments with a high density of electronics, there can benumerous sources of EMI, and these EMI sources can be significant. Theelectromagnetic disturbances can also occur at substantially a singlefrequency, which upon sampling can get folded into a DC component. Theseelectromagnetic disturbances can land on top of a desired sensor signaland obliterate the desired signal. For example, if a desired signal issampled at a 100 kHz clock frequency, and the disturbance is at 100 kHz,then when sampling the disturbance at the clock frequency it can appearas a substantially DC signal. Thus, it is important to protect desiredsensor signals, especially along capacitive paths, from EMI. The EMIproblem is especially important to solve in safety critical applicationsthat are in harsh environments, for example the sensors used forelectronic stability in an automobile.

The two commonly used solutions to EMI are shielding the sensor withmetal, and using a differential approach. Shielding the sensor withmetal includes creating a Faraday cage to block external electric fieldswhich can cause EMI. However, shielding can be bulky and expensive,especially when there are numerous sensors to be shielded or there is ahigh density of electronics to be fit in a small area.

The differential approach takes the differences between signals onparallel wires which can substantially subtract out the electromagneticdisturbance as a common mode signal. FIG. 2 illustrates the differentialapproach. The exemplary differential sensor and amplifier system 200includes a MEMS device 220 coupled to an ASIC 240 by bond wires 260,262. Each of the bond wires 260, 262 experiences EMI from external EMIsources 210. There is capacitive coupling 250 between the EMI source 210and the first bond wire 260 creating a first disturbance capacitance C1,and there is capacitive coupling 252 between the EMI source 210 and thesecond bond wire 262 creating a second disturbance capacitance C2. Ifthe disturbance capacitances C1 and C2 between the EMI sources 210 andthe bond wires 260, 262 are the same, then the electromagneticdisturbance is rejected due to the common mode rejection of thedifferential amplifier of the ASIC 240. However, in order to achieve thedesired cancellation, the disturbance capacitances C1 and C2 between theEMI sources 210 and the bond wires 260, 262 should not be mismatched bymore than 0.5%. This matching can be very difficult to achieve inpractice. Even if the matching is achieved initially, bond wires can bedisturbed or warped, for example by an automobile accident. Thismovement of the bond wires can cause asymmetry between the bond wires,which can cause an unwanted mismatch in the disturbance capacitances andreduce the effectiveness of the differential approach.

It would be desirable to have a robust technique for reducingelectromagnetic interference that also overcomes some of thedisadvantages of shielding and differential circuits.

SUMMARY OF THE INVENTION

A capacitive sensor system resistant to electromagnetic interference isdisclosed that includes a capacitive core, a differential amplifier,first and second capacitive paths, and a chopping system. The capacitivecore includes a first variable capacitor, a second variable capacitor, afirst core output coupled to the first variable capacitor, a second coreoutput coupled to the second variable capacitor, and a common nodecoupling the first variable capacitor and the second variable capacitor.The differential amplifier includes an inverting input and anon-inverting input. The first capacitive path couples the first coreoutput to the inputs of the differential amplifier, and the secondcapacitive path couples the second core output to the inputs of thedifferential amplifier. The chopping system has a high state and a lowstate, and couples the first and second core outputs to the inputs ofthe differential amplifier. When the chopping system is in the highstate, a positive step voltage is applied to the common node of thecapacitive core, and the chopping system couples the first core outputto the inverting input of the differential amplifier and couples thesecond core output to the non-inverting input of the differentialamplifier. When the chopping system is in the low state, a negative stepvoltage is applied to the common node of the capacitive core, and thechopping system couples the first core output to the non-inverting inputof the differential amplifier and couples the second core output to theinverting input of the differential amplifier. The capacitive core canbe a microelectromechanical device. The first capacitive path caninclude a first bond wire, and the second capacitive path can include asecond bond wire. The chopping system can be varied between the highstate and the low state at frequencies that smear noise away from afrequency band of interest, or that smear noise substantially evenlyacross a wide frequency range, or at random frequencies.

A capacitive sensor system resistant to electromagnetic interference isdisclosed that includes first and second capacitive cores, adifferential amplifier, first and second capacitive paths, and achopping system. The first capacitive core includes a first variablecapacitor, a second variable capacitor, a first core input coupled tothe first variable capacitor, a second core input coupled to the secondvariable capacitor, and a first common node coupling the first variablecapacitor and the second variable capacitor. The second capacitive coreincludes a third variable capacitor, a fourth variable capacitor, athird core input coupled to the third variable capacitor, a fourth coreinput coupled to the fourth variable capacitor, and a second common nodecoupling the third variable capacitor and the fourth variable capacitor.The differential amplifier includes an inverting input and anon-inverting input. The first capacitive path couples the first commonnode and the inputs of the differential amplifier, and the secondcapacitive path couples the second common node and the inputs of thedifferential amplifier. The chopping system has a high state and a lowstate, and couples the first and second common nodes to the inputs ofthe differential amplifier. When the chopping system is in the highstate, a positive reference voltage is applied to the first input of thefirst capacitive core and to the fourth input of the second capacitivecore, a negative reference voltage is applied to the second input of thefirst capacitive core and to the third input of the second capacitivecore, and the chopping system couples the first common node to theinverting input of the differential amplifier and couples the secondcommon node to the non-inverting input of the differential amplifier;the negative reference voltage being substantially the same magnitudeand opposite polarity as the positive reference voltage. When thechopping system is in the low state, a negative reference voltage isapplied to the first input of the first capacitive core and to thefourth input of the second capacitive core, a positive reference voltageis applied to the second input of the first capacitive core and to thethird input of the second capacitive core, and the chopping systemcouples the first common node to the non-inverting input of thedifferential amplifier and couples the second common node to theinverting input of the differential amplifier. The first and secondcapacitive cores can be part of a microelectromechanical device. Thefirst capacitive path can include a first bond wire and the secondcapacitive path can include a second bond wire. The chopping system canbe varied between the high state and the low state at frequencies thatsmear noise away from a frequency band of interest, or that smear noisesubstantially evenly across a wide frequency range, or at randomfrequencies.

A method of making a capacitive sensor system resistant toelectromagnetic interference is disclosed. The method includesswitchably coupling a first output of a capacitive sensor to inputs of adifferential amplifier, switchably coupling a second output of thecapacitive sensor to the inputs of the differential amplifier, andflipping a chopping system between a high state and a low state tocontrol electromagnetic interference. In this method, the differentialamplifier includes an inverting input and a non-inverting input, and thesecond output of the capacitive sensor is different from the firstoutput of the capacitive sensor. When the chopping system is in the highstate, the method also includes applying a first polarity voltage to aninput of the capacitive sensor, coupling the first output of thecapacitive sensor to the inverting input of the differential amplifier,and coupling the second output of the capacitive sensor to thenon-inverting input of the differential amplifier. When the choppingsystem is in the low state, the method also includes applying a secondpolarity voltage to the input of the capacitive sensor, the secondpolarity voltage having substantially the same magnitude and oppositepolarity as the first polarity voltage, coupling the first output of thecapacitive sensor to the non-inverting input of the differentialamplifier; and coupling the second output of the capacitive sensor tothe inverting input of the differential amplifier. The flipping step caninclude flipping the chopping system between the high state and the lowstate at frequencies that smear noise away from a frequency band ofinterest, or that smear noise substantially evenly across a widefrequency range, or at random frequencies.

The method can be done using a capacitive sensor that includes acapacitive core with a first variable capacitor, a second variablecapacitor, a first core output coupled to the first variable capacitor,a second core output coupled to the second variable capacitor, and acommon node coupling the first variable capacitor and the secondvariable capacitor. In this case, the first core output is the firstoutput of the capacitive sensor, the second core output is the secondoutput of the capacitive sensor, and the common node is the input of thecapacitive sensor. The method can also be done using a capacitive sensorthat includes first and second capacitive cores, where the firstcapacitive core includes a first variable capacitor, a second variablecapacitor, a first core input coupled to the first variable capacitor, asecond core input coupled to the second variable capacitor, and a firstcommon node coupling the first variable capacitor and the secondvariable capacitor; and the second capacitive core includes a thirdvariable capacitor, a fourth variable capacitor, a third core inputcoupled to the third variable capacitor, a fourth core input coupled tothe fourth variable capacitor, and a second common node coupling thethird variable capacitor and the fourth variable capacitor. In thiscase, the first output of the capacitive sensor is the first common nodeof the first capacitive core, the second output of the capacitive sensoris the second common node of the second capacitive core, and the inputof the capacitive sensor be any of the first and second core inputs ofthe first capacitive core and the third and fourth core inputs of thesecond capacitive core.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features and objects of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 illustrates electromagnetic disturbance or interference (EMI) dueto capacitive coupling between bond wires and nearby cables, plates,circuitry, etc.;

FIG. 2 illustrates a differential approach to overcome electromagneticdisturbances;

FIG. 3 illustrates an exemplary differential sensor and amplifier systemthat can implement a robust technique for reducing electromagneticinterference;

FIG. 4 shows how a chopping pattern can be used to reduce anelectromagnetic disturbance by smearing it across a wide frequencyrange;

FIG. 5A shows an exemplary full-bridge differential capacitiveaccelerometer when the chopping signal is in the low state;

FIG. 5B shows the exemplary full-bridge differential accelerometer ofFIG. 5A when the chopping signal is in the high state;

FIG. 6 shows how a shaped chopping pattern can be used to smear theerror due to the offset difference in the two chop states away from DCas shaped noise; and

FIG. 7 shows a potential tradeoff between a shaped chopping pattern andan unshaped random pattern.

Corresponding reference characters indicate corresponding partsthroughout the several views. Although the exemplification set outherein illustrates embodiments of the invention, in several forms, theembodiments disclosed below are not intended to be exhaustive or to beconstrued as limiting the scope of the invention to the precise formsdisclosed.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 3 illustrates an exemplary differential sensor and amplifier system300 that can implement a robust technique for reducing electromagneticinterference that overcomes several disadvantages of shielding anddifferential circuits. The exemplary differential sensor and amplifiersystem 300 includes a MEMS device 320 coupled to an ASIC 340 by bondwires 360, 362. The ASIC 340 includes a differential amplifier 342 and achopping system 344. The first bond wire 360 connects a first output 51of the MEMS device 320 to one side of the differential amplifier 342,and the second bond wire 362 couples a second output of the MEMS device320 to the other side of the differential amplifier 342. There can beadditional bond wires and connections between the MEMS device 320 andthe ASIC 340 but only two are shown for clarity. The ASIC 340 can alsoinclude additional circuitry. Each of the bond wires 360, 362experiences EMI from external EMI sources 310. There is capacitivecoupling 350 between the EMI source 310 and the first bond wire 360creating a first disturbance capacitance C1, and there is capacitivecoupling 352 between the EMI source 310 and the second bond wire 362creating a second disturbance capacitance C2.

The circuit of FIG. 3 is like the circuit of FIG. 2 except that choppingsystem 344 has been added. The chopping system 344 swaps the connectionsbetween outputs S1, S2 of the MEMS device 320 and the inputs of thedifferential amplifier 342, which in effect flips the bond wires 360,362. When the chopping signal Φ_(ch) is in the low state, the switchesof the chopping system 344 couple the first output S1 of the MEMS device320 to the inverting input of the differential amplifier 342, and couplethe second output S2 of the MEMS device 320 to the non-inverting inputof the differential amplifier 342. During this low phase, a positivestep voltage is used to excite the sensor 320 which causes the chargethat flows into the amplifier 342 to be:

Q _(in) =VS*(ΔC _(sensor))+ΔV _(emc)*(C1−C2)  (1)

where Q_(in) is the charge input to the amplifier 342, Vs is the stepvoltage that excites the sensor 320, ΔC_(sensor) is the differentialcharge of the variable capacitors of the sensor 320, ΔV_(emc) is theelectromagnetic interference between the bond wires 360, 362 and the EMIsource 310, and C1 and C2 are the disturbance capacitances.

When the chopping signal Φ_(ch) is in the high state, the switches ofthe chopping system 344 couple the second output S2 of the MEMS device320 to the inverting input of the differential amplifier 342, and couplethe first output S1 of the MEMS device 320 to the non-inverting input ofthe differential amplifier 342. During this high phase, a negative stepvoltage is used to excite the sensor 320 which causes the charge thatflows into the amplifier 342 to be:

$\begin{matrix}\begin{matrix}{Q_{in} = {{\left( {- {Vs}} \right)*\left( {\Delta \; C_{sensor}} \right)*\left( {- 1} \right)} + {\Delta \; V_{emc}*\left( {{C\; 2} - {C\; 1}} \right)}}} \\{= {{{Vs}*\left( {\Delta \; C_{sensor}} \right)} - {\Delta \; V_{emc}*\left( {{C\; 1} - {C\; 2}} \right)}}}\end{matrix} & (2)\end{matrix}$

Comparing Eq. (1) and Eq. (2), it can be seen that the first term due tothe sensor signal is the same while the second term due to theelectromagnetic disturbance reverses sign. Thus, as the chopping signalflips back and forth between low and high states, the desired signal isunchanged but the polarity of the electromagnetic disturbance flips backand forth. By using a pattern for the chopping signal, electromagneticinterference can be smeared across a wide frequency range or smearedaway from a particular frequency band.

If we do not know the frequency of the electromagnetic disturbance, arandom pattern can be used to smear the electromagnetic disturbanceacross a wide frequency range. FIG. 4 shows the smearing of theelectromagnetic disturbance across a wide frequency range. In FIG. 4,the top plots are in the time domain and the bottom plots are in thefrequency domain. FIG. 4A1 shows a random pattern for the choppingsignal Φ_(ch) in the time domain, and FIG. 4A2 shows the random choppingsignal spread across a wide range in the frequency domain. The energy ofthe random chopping pattern is distributed substantially equally acrossfrequencies. FIG. 4B1 shows an exemplary sinusoidal electromagneticdisturbance (ΔV_(emc)) in the time domain, and FIG. 4B2 shows theexemplary electromagnetic disturbance in the frequency domain. Theenergy of the exemplary electromagnetic disturbance is concentrated at asingle frequency. FIG. 4C1 shows the result of combining the randomchopping signal with the exemplary electromagnetic disturbance in thetime domain, and FIG. 4C2 shows the result of combining these twosignals in the frequency domain. The energy of the resulting disturbancesignal is smeared substantially equally across a wide frequency range.

This technique can achieve a significant improvement in dealing withelectromagnetic disturbances. As shown in FIG. 4, a very largeelectromagnetic disturbance at a single frequency can be distributedacross a wide frequency range. For example, by using a clock frequencyof 1 MHz and a desired bandwidth of 50 Hz, this technique provides animprovement in electromagnetic robustness of 10 log(1 MHz/(50 Hz*2))=40dB which is a significant benefit.

This EMI robustness technique can be used in any capacitive sensor, forexample, it can be used in gyroscopes, accelerometers, pressure sensorsetc. FIG. 3 shows an exemplary half-bridge (two capacitors) capacitivesensor. FIG. 5 shows an exemplary full-bridge (four capacitors)differential capacitive accelerometer. FIG. 5A shows the couplings whenthe chopping signal is in the low state and FIG. 5B shows the couplingswhen the chopping signal is in the high state. A switching system likethe chopping system 344 of FIG. 3 can be used here, but the couplings ineach state are explicitly shown for clarity. Note that it is thecapacitive nodes like those between the capacitive core outputs of theMEMS and the amplifier inputs that are affected by the electromagneticdisturbances and capacitive coupling.

In the exemplary embodiment of FIG. 5, the MEMS includes a firstcapacitive core CA and a second capacitive core CB. The first capacitivecore CA includes a first variable capacitor CA1 and a second variablecapacitor CA2 that are coupled by a first common node M1. The firstcapacitive core CA also includes a first input coupled to the firstvariable capacitor CA1 and a second input coupled to the second variablecapacitor CA2. The second capacitive core CB includes a third variablecapacitor CB1 and a fourth variable capacitor CB2 that are coupled by asecond common node M2. The second capacitive core CB also includes athird input coupled to the third variable capacitor CB1 and a fourthinput coupled to the fourth variable capacitor CB2. The first commonnode M1 of the first capacitive core CA is coupled to an upper MEMSoutput, and the second common node M2 of the second capacitive core CBis coupled to a lower MEMS output.

In FIG. 5A, when the chopping signal is in the low state, the upper MEMSoutput is coupled to the inverting input of the ASIC amplifier and thelower MEMS output is coupled to the non-inverting input of the ASICamplifier. Also when the chopping signal is in the low state, a positivereference voltage +Vs is applied to the first input of the firstcapacitive core CA and to the fourth input of the second capacitive coreCB, and a negative reference voltage −Vs is applied to the second inputof the first capacitive core CA and to the third input of the secondcapacitive core CB. The positive and negative reference voltages havingsubstantially the same magnitude and opposite polarities.

In FIG. 5B, when the chopping signal is in the high state, the MEMSoutputs and the reference voltages are flipped. When the chopping signalis in the high state, the upper MEMS output is coupled to thenon-inverting input of the ASIC amplifier and the lower MEMS output iscoupled to the inverting input of the ASIC amplifier. Also when thechopping signal is in the high state, a negative reference voltage −Vsis applied to the first input of the first capacitive core CA and to thefourth input of the second capacitive core CB, and a positive referencevoltage +Vs is applied to the second input of the first capacitive coreCA and to the third input of the second capacitive core CB.

The shape of the chopping pattern can be selected to achieve the rightcompromise between EMI robustness and tolerance to MEMS non-idealities.In some cases, a flat spectrum chopping sequence like that shown in FIG.4A1 and 4A2 may not be the best choice. For example, if due to sensornon-idealities (for example parasitic capacitances), the offsets in thelow and high phases of the chop signal are different, then it may bebetter to use a shaped chopping sequence. Plain random chopping smearsthe offset difference as white noise which puts some noise around DC andraises the noise floor. A shaped chopping sequence can be used to smearthe noise away from a particular frequency band. For example, if thefrequency band of interest is at DC or low frequencies, a shapedchopping sequence can be used that smears the noise to higherfrequencies.

FIG. 6 shows how a shaped chopping pattern can be used to smear theerror due to the offset difference in the two chop states away from DCas shaped noise. FIG. 6A shows a chopping pattern in the frequencydomain. The chopping pattern has substantially no DC or low frequencycomponent and starts ramping up at higher frequencies. FIG. 6B shows theDC error due to the difference in the offsets between the high chopstate and the low chop state. FIG. 6C shows the result in the frequencydomain of combining the shaped chopping pattern of FIG. 6A with the DCerror due to the offset difference of FIG. 6B. The error in output dueto the offset difference is shaped as noise away from DC and lowfrequencies, the frequency band of interest, and into higherfrequencies.

However, the use of a shaped pattern can result in slightly more EMIinduced disturbance for certain EMI frequencies. FIG. 7 illustrates thepotential tradeoff between a shaped chopping pattern and an unshapedrandom pattern. FIG. 7 shows the frequency spectrum of an unshapedrandom chopping pattern 702 and of an exemplary shaped chopping pattern704. If the aliased EMI frequency is less than frequency fa, for exampleat frequency f_(emi1), then the shaped pattern 704 folds less noise ontoDC than the unshaped pattern 702. However, if the aliased EMI frequencyis greater than frequency fa, for example at frequency f_(emi2), thenthe unshaped pattern 702 folds less noise onto DC than the shapedpattern 704. System level considerations can be used to decide thedesired chopping pattern.

While this invention has been described as having an exemplary design,the present invention may be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles.

1. A capacitive sensor system resistant to electromagnetic interference,the capacitive sensor system comprising: a capacitive core including afirst variable capacitor, a second variable capacitor, a first coreoutput coupled to the first variable capacitor, a second core outputcoupled to the second variable capacitor, and a common node coupling thefirst variable capacitor and the second variable capacitor; adifferential amplifier including an inverting input and a non-invertinginput; a first capacitive path between the first core output and theinputs of the differential amplifier; a second capacitive path betweenthe second core output and the inputs of the differential amplifier; achopping system having a high state and a low state, the chopping systemcoupling the first and second core outputs to the inputs of thedifferential amplifier, wherein when the chopping system is in the highstate, a positive step voltage is applied to the common node of thecapacitive core, and the chopping system couples the first core outputto the inverting input of the differential amplifier and couples thesecond core output to the non-inverting input of the differentialamplifier; and when the chopping system is in the low state, a negativestep voltage is applied to the common node of the capacitive core, andthe chopping system couples the first core output to the non-invertinginput of the differential amplifier and couples the second core outputto the inverting input of the differential amplifier.
 2. The capacitivesensor system of claim 1, wherein the capacitive core is amicroelectromechanical device.
 3. The capacitive sensor system of claim2, wherein the first capacitive path includes a first bond wire and thesecond capacitive path includes a second bond wire.
 4. The capacitivesensor system of claim 1, wherein the first capacitive path includes afirst bond wire and the second capacitive path includes a second bondwire.
 5. The capacitive sensor system of claim 1, wherein the choppingsystem is varied between the high state and the low state at frequenciesthat smears noise away from a frequency band of interest.
 6. Thecapacitive sensor system of claim 1, wherein the chopping system isvaried between the high state and the low state at frequencies thatsmears noise substantially evenly across a wide frequency range.
 7. Thecapacitive sensor system of claim 6, wherein the chopping system isvaried between the high state and the low state at random frequencies.8. A capacitive sensor system resistant to electromagnetic interference,the capacitive sensor system comprising: a first capacitive coreincluding a first variable capacitor, a second variable capacitor, afirst core input coupled to the first variable capacitor, a second coreinput coupled to the second variable capacitor, and a first common nodecoupling the first variable capacitor and the second variable capacitor;a second capacitive core including a third variable capacitor, a fourthvariable capacitor, a third core input coupled to the third variablecapacitor, a fourth core input coupled to the fourth variable capacitor,and a second common node coupling the third variable capacitor and thefourth variable capacitor; a differential amplifier including aninverting input and a non-inverting input; a first capacitive pathbetween the first common node and the inputs of the differentialamplifier; a second capacitive path between the second common node andthe inputs of the differential amplifier; a chopping system having ahigh state and a low state, the chopping system coupling the first andsecond common nodes to the inputs of the differential amplifier, whereinwhen the chopping system is in the high state, a positive referencevoltage is applied to the first input of the first capacitive core andto the fourth input of the second capacitive core, a negative referencevoltage is applied to the second input of the first capacitive core andto the third input of the second capacitive core, and the choppingsystem couples the first common node to the inverting input of thedifferential amplifier and couples the second common node to thenon-inverting input of the differential amplifier; the negativereference voltage being substantially the same magnitude and oppositepolarity as the positive reference voltage; and when the chopping systemis in the low state, a negative reference voltage is applied to thefirst input of the first capacitive core and to the fourth input of thesecond capacitive core, a positive reference voltage is applied to thesecond input of the first capacitive core and to the third input of thesecond capacitive core, and the chopping system couples the first commonnode to the non-inverting input of the differential amplifier andcouples the second common node to the inverting input of thedifferential amplifier.
 9. The capacitive sensor system of claim 8,wherein the first and second capacitive cores are part of amicroelectromechanical device.
 10. The capacitive sensor system of claim9, wherein the first capacitive path includes a first bond wire and thesecond capacitive path includes a second bond wire.
 11. The capacitivesensor system of claim 8, wherein the first capacitive path includes afirst bond wire and the second capacitive path includes a second bondwire.
 12. The capacitive sensor system of claim 8, wherein the choppingsystem is varied between the high state and the low state at frequenciesthat smears noise away from a frequency band of interest.
 13. Thecapacitive sensor system of claim 8, wherein the chopping system isvaried between the high state and the low state at frequencies thatsmears noise substantially evenly across a wide frequency range.
 14. Thecapacitive sensor system of claim 13, wherein the chopping system isvaried between the high state and the low state at random frequencies.15. A method of making a capacitive sensor system resistant toelectromagnetic interference, the method comprising: switchably couplinga first output of a capacitive sensor to inputs of a differentialamplifier, the differential amplifier including an inverting input and anon-inverting input; switchably coupling a second output of thecapacitive sensor to the inputs of the differential amplifier, thesecond output of the capacitive sensor being different from the firstoutput of the capacitive sensor; flipping a chopping system between ahigh state and a low state to control electromagnetic interference, whenthe chopping system is in the high state: applying a first polarityvoltage to an input of the capacitive sensor, coupling the first outputof the capacitive sensor to the inverting input of the differentialamplifier; and coupling the second output of the capacitive sensor tothe non-inverting input of the differential amplifier; and when thechopping system is in the low state, applying a second polarity voltageto the input of the capacitive sensor, the second polarity voltagehaving substantially the same magnitude and opposite polarity as thefirst polarity voltage; coupling the first output of the capacitivesensor to the non-inverting input of the differential amplifier; andcoupling the second output of the capacitive sensor to the invertinginput of the differential amplifier.
 16. The method of claim 15, whereinthe flipping step comprises flipping the chopping system between thehigh state and the low state at frequencies that smear noise away from afrequency band of interest.
 17. The method of claim 15, wherein theflipping step comprises flipping the chopping system between the highstate and the low state at frequencies that smear noise substantiallyevenly across a wide frequency range.
 18. The method of claim 17,wherein the flipping step comprises flipping the chopping system betweenthe high state and the low state at random frequencies.
 19. The methodof claim 15, wherein the capacitive sensor comprises a capacitive coreincluding a first variable capacitor, a second variable capacitor, afirst core output coupled to the first variable capacitor, a second coreoutput coupled to the second variable capacitor, and a common nodecoupling the first variable capacitor and the second variable capacitor;the first core output being the first output of the capacitive sensor,the second core output being the second output of the capacitive sensor,and the common node being the input of the capacitive sensor.
 20. Themethod of claim 15, wherein the capacitive sensor comprises a firstcapacitive core and a second capacitive core; the first capacitive coreincluding a first variable capacitor, a second variable capacitor, afirst core input coupled to the first variable capacitor, a second coreinput coupled to the second variable capacitor, and a first common nodecoupling the first variable capacitor and the second variable capacitor;the second capacitive core including a third variable capacitor, afourth variable capacitor, a third core input coupled to the thirdvariable capacitor, a fourth core input coupled to the fourth variablecapacitor, and a second common node coupling the third variablecapacitor and the fourth variable capacitor; the first output of thecapacitive sensor being the first common node of the first capacitivecore, the second output of the capacitive sensor being the second commonnode of the second capacitive core, and the input of the capacitivesensor being any of the first and second core inputs of the firstcapacitive core and the third and fourth core inputs of the secondcapacitive core.